Undelivered email

If a message you sent cannot be delivered, we will email you to let you know. If you're not sure why it wasn't delivered, this document will help you decode the message.

Undelivered mail returned to sender

If you received a message back from FastMail, you should read the message to see what it says. The message should look something like this:

This is the Postfix program at host www.fastmail.com. I'm sorry to have to inform you that the message returned below could not be delivered to one or more destinations. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the message returned below.

In this case, the other end refused our connection. This could be because the system is down, or because they are specifically blocking FastMail. You should try sending the email again in a day or two, and if it continues to fail, contact the administrator of the other system (usually postmaster@domain.tld) to find out what is happening. You will probably have to do this from another account, since you probably won't be able to send from FastMail to the support or webmaster address either.

When you see the message host xxx said:, it means we got an error response from the other server. The lines after this are what we got back from the other end, so you should contact the administrator of the other system if you don't understand what they mean (usually postmaster@domain.tld).

In this case, the response was "user unknown", which usually means that the email address doesn't exist on the remote system.